The invention relates to a process which is suitable for applying a permanently adhering, stable, dirt and water-repellent coating to metallic surfaces, specifically chromium surfaces, specifically on sanitary and kitchen fixtures, as well as components coated in this manner.
Water fixtures in the sanitary field are generally in frequent use every day and are always in the direct view of the user. For these two reasons they have to be cleaned regularly, since contaminants on the surface such as calcium around edges, leftover dirt, cream, soap, toothpaste, etc. and fingerprints spoil the visual impression. In addition to the substantial expenditure of labor, regular cleaning is accompanied by the use of environmentally polluting cleaning agents and mechanical stress on the surface of the fixtures from the use of abrasive cleaners. The visually immaculate impression of a newly cleaned fixture is usually lost at the next subsequent use.
Modern decorative surfaces (as for example, in the sanitary field) are distinguished by the fact that they demonstrate multi-functional coating properties in addition to their decorative appeal. For example, among these functional coating properties are the anti-adhesive characteristics of surfaces. Surfaces of this type possess great resistance to being further covered, for example, by dirt particles or paints. Because of the anti-adhesive character of these surfaces, these coatings also have low sensitivity to fingerprints which can occur during production, installation or in the daily use of sanitary fixtures. Since anti-adhesive surfaces have a hydrophobic character, these coatings usually possess higher resistance to corrosion.
Anti-adhesive, dirt-repellent properties can be achieved, for example, by coating a galvanically chrome-plated surface (e.g. a bath fixture) with an anti-adhesive coating (e.g. a sol-gel coat). The integrity of these coating systems is, in addition to the properties of the coat, fundamentally dependent on the adhesion of the coat to the chromium surface. Since the chromium surface is present in very different, or non-defined, states as a result of production restraints, no process is currently known that is suitable for applying a sol-gel system to adhere firmly to a chromium surface.
The structure of a galvanically deposited chromium coat consists of a copper base coat, a nickel intermediate coat and a chromium top coat. These coats are applied galvanically one after the other. These production steps are supplemented by numerous activation and rinse treatments between the individual coating steps. The condition of the surface obtained by the coating is therefore a function resulting both from the physical and chemical properties of the coating material as well from the type of coating chemicals employed.
If a newly deposited chromium surface is exposed to normal atmosphere, a closed, passivating chromium oxide layer of several layers of atoms forms on the surface of the chromium coat. This oxide layer prevents further oxidation of the chromium underneath it and is one of the causes of poorer wetting characteristics with respect to high-polar liquids, so that normally problems arise regarding wetting and adhesion strength when a chromium surface undergoes additional coating. Thus water on a smooth, galvanically deposited chromium coat has a wetting angle of 90°, a typical value for hydrophobic surfaces, which do not permit wetting by media with polar groups.
Currently, two primary concepts are pursued in the production of surfaces having a dirt-repellent action:
Firstly, the application of a surface coating whose outermost surface has the lowest possible surface tension and thus a minimal tendency for contaminants to adhere.
Secondly, texturing the surface with peaks and valleys in the millimicron and micron range which result in water beading easily, whereby any contaminant can be removed using water (“lotus effect”).
Because of the texture, the latter concept does not permit smooth, shiny surfaces, such as have been widespread in metal fixtures for decades and are expected by customers. The micro-structures described are additionally not very stable mechanically, as a result of which a gradual deterioration in the dirt-repellent effect can be expected. For these reasons, the former concept was pursued in the present invention.
The coating materials under consideration here are on the one hand conventional organic paints with surface tension-reducing additives such as silicon, on the other fluoro-organically functionalized sol-gel coatings and additionally perfluorinated polymers such as poly(tetrafluoroethylene).
The first-named coating materials generally have to be applied at a relatively high film thickness (30 to several hundred microns), they are mostly of limited chemical and mechanical stability and generally do not have extremely low surface tension, so that no decisive reduction in sensitivity to dirt is achieved compared with chromium.
The perfluoropolymers mentioned also have to be applied at a high film thickness (mostly more than 100 microns). Working against the advantages of high chemical stability and their pronounced anti-adhesive action are the additional disadvantages that the formation of a closed coat after the application of the polymer dispersion does not take place until very high temperatures (about 300° C. and higher), that the mechanical hardness of the coats is low and that for the most part transparent coats are not achieved, but only dull ones.
One process for producing mechanically stable and highly anti-adhesive surfaces, which is described many times over in the patent literature (e.g. in WO 9842886, U.S. Pat. No. 5,753,313, CN 1077144), lies in two-coat systems, consisting of a thermally sprayed (or electric-arc sprayed) ceramic or metallic coat and a subsequently applied coat of silicon resin or, better, fluoropolymer, which both covers the surface of the sprayed coat and also fills its valleys and pores. This process is costly overall, since it contains two expensive coating steps involving completely different technologies and is reserved for temperature-stable substrates because of the heat of the spray material and the high spraying temperatures for the polymer resins. Furthermore, the result is textured, non-transparent surfaces.
Polysiloxanes produced by the sol-gel process are also used as the base coat between the substrate and the fluoropolymer(JP 06145946). As a result, the temperature stress is certainly less when the base coat is applied, but the mechanical sensitivity of the overlying polymer resin is not improved thereby, and the adhesion of the sol-gel coat (and consequently of the entire composite coat) on substrates such as chromium is inadequate.
As an alternative, a perfluoropolymer phase in the form of an IPN (interpenetrating network) or a nanocomposite with a different polymer, e.g. a polysiloxane (as disclosed in WO 9701599) can be applied. Materials of this kind lead one to expect good coverage of the substrate on account of low surface tension, but the problem of adhesion on a smooth surface, e.g. of chromium, is similarly still not solved thereby.
Sol-gel coatings possess the advantage of forming stable, transparent coats even at clearly lesser film thicknesses (1-10 microns). This consumes less coating material, and detracts minimally from the external appearance of the coating on the article. The crosslinking of coats of this type takes places at temperatures as low as between 100° C. and 150° C., reducing energy consumption and also allowing thermally sensitive substrates (e.g. galvanically chrome-plated plastics) to be coated without damage. Because of their high degree of cross-linking these coats possess a mechanical stability which is superior to that of organic materials. The high inorganic content in compounds of this type also results in high stability against chemical attack and high temperatures. The stable incorporation of perfluoro-organic groups into the surface of a coating of this type results in surface tensions which are still lower than those of current perfluorinated polymers (about 18 mN/m), although the percentage by mass of the perfluoro chemicals in the overall mixture is very much lower.
These types of systems of fluoro-organic functionalized nanoparticle sol coatings are known from numerous patents, such as DE 2446279, JP 06145600, WO 92/21729, DE 19917367, DE 10004132.
Because of the properties profile described, sol-gel coatings of this type seem perfect for creating dirt and water-repellent coatings on sanitary fixtures, specifically chrome-plated sanitary fixtures, since a powerful anti-stick effect can be achieved without the loss of the beneficial properties of the metallic surfaces. The previously unsolved problem in this problem was the too low surface tension of galvanically created chromium surfaces which resulted in poor wetting and too weak adhesion.